This invention relates to the SZ twisting of twist elements of electric cables and lines in general, and more particularty to reducing the influence of the reversal points of the twist direction of the twist elements on the mechanical and/or electrical properties of the material to be twisted.
The development in the field of SZ twisting technology for electric cables and lines, which started at the beginning of the 1960s, has led to SZ twisting methods and SZ twisting machines, with which two to five twist elements can be twisted together without problem. Today, SZ twisting is used in the combined pair, triple or star quad base unit twisting of communication cables as well as for the twisting, with subsequent jacketing, of multiconductor power cables with conductor cross sections of up to 2.5 mm.sup.2 which are to be permanently installed.
The use of SZ twisting is considered wherever a twisting operation and another operation can be combined. Besides the just mentioned fabrication cycles, this also applies to the twisting of flexible elements in combination with another operation, for instance, to the twisting and subsequent jacketing of the conductors of flexible lines, the conductors of which consist of stranded wire, or also to the twisting of litz conductors and the subsequent application (extrusion) of the insulation. Such flexible lines are, for instance, pairs, power cords for home appliances and connecting cords for telephone equipment.
The SZ twisting of the twist elements customary in practice is as a rule performed by means of accumulators, in which the material to be twisted, which has been assembled into a strand, is conducted in a stretched condition. The apparatus used consists of a first twisting point, e.g., a twisting closer, arranged at the entrance to the accumulator, and of a second twisting point (for instance, a twisting closer or a deflection pulley) arranged at the exit of the accumulator, and rotatable twisting heads which are arranged immediately behind the first twisting point and immediately ahead of the second twisting point, respectively. The axes of rotation of the twisting heads lie on the twisting axis, along which the twist elements assembled in a strand are also conducted. For the purpose of SZ twisting, either the direction of rotation of the two twisting heads or, with the direction of rotation remaining constant, only the speed of rotation of one or both twisting heads is varied at intervals which correspond to the running time of a cross section element of the material to be twisted from the first twisting point to the second twisting point (German Auglegeschrift No. 15 15 731; German Pat. No. 22 30 972). The change of the rotary movement of the twisting heads, as a rule, takes plae periodically in two steps, i.e., the rotary motion changes between clockwise and counterclockwise motion or between a low and a high speed of rotation. However, it is also known to vary the rotary motion of an accumulator constructed as a pulley accumulator in several steps within one twisting period. The distance between the reversal points of the twist direction in the finished twisted material can thereby be made larger than the storage length of the accumulator (German Offenlegungsschrift No. 18 01 055).
In the art of SZ twisting machines which have been developed more recently, and are now in use, in which the material to be twisted is conducted through the accumulator in stretched condition, the distance between the entrance and the exit of the accumulator and, therefore, between the twisting heads arranged at the entrance and the exit is about 2 to 10m. Since the rotary motion of the twisting heads is changed at intervals which correspond to the running time of a cross section element of the material through the accumulator, the distance between the reversal points of the twist direction in the finished twisted material is also about 2 to 10 m. These reversal points of the twist direction are mechanical as well as electrical weak points, since, on the one hand, the twist elements can untwist in the vicinity of the reversal points, and on the other hand, since the flexibility of the twist elements is reduced in the vicinity of the reversal points, and because the twist elements are arranged relatively untwisted relative to each other in the vicinity of the reversal points, a particularly strong electric coupling results there in communication cables. A number of measures is known which are aimed at reducing the negative effects of these reversal points while retaining the above-mentioned distance between the reversal points of the twist direction. For instance, it is known to twist the twist elements with a relatively short length of lay in the vicinity of the reversal points, or to let the distance between reversal points vary periodically about a mean value. However, it is also known to increase the distance between the reversal points of the twist direction considerably by selecting an accumulator with a very large storage length. The mechanical design proposed for this purpose provides a winding member, disposed coaxially with the twisting axis and oscillating about the latter, on which the material to be twisted is arranged in a multiplicity of turns around the twisting axis, and with which guide members are associated on both sides, with one winding the material to be twisted on the winding member and the other unwinding the material to be twisted from the winding member. The winding member itself is designed so that the turn of the material to be twisted, which has been put on at the one end, can be transported along the winding body to the other end thereof (German Offenlegungsschrift No. 16 85 842). Twisted material made on such an accumulator, i.e., material in which the reversal points of the twist direction have a large mutual distance, can be cut into short use lengths which have no reversal points of the twist direction. The design of the accumulator provided for this purpose is very elaborate. However, its design with respect to fixing those points of the material to be twisted, at which the winding direction of the storage element is changed, appears difficult, and the production speed would seem to be relatively low.